Catalysts for producing narrow carbon nanostructures

ABSTRACT

A method for producing high yields of high-purity carbon nanostructures having uniform average widths narrower than conventional carbon nanostructures. The nanostructures are produced from unsupported catalytic metal powders. A dispersing agent, such as sodium chloride, is blended with the catalytic metal powders prior to milling to the desired catalytic size to prevent the powder particles from sintering.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This present invention relates to a method for producing highyields of high-purity carbon nanostructures having uniform averagewidths narrower than conventional carbon nanostructures. Thenanostructures are produced from unsupported catalytic metal powders. Adispersing agent, such as sodium chloride, is blended with the catalyticmetal powders prior to milling to the desired catalytic size to preventthe powder particles from sintering.

[0003] 2. Description of Related Art

[0004] Nanostructure materials, particularly carbon nanostructurematerials, are quickly gaining importance for various potentialcommercial applications. Such applications include their use to storehydrogen, to serve as catalyst supports, to be useful in variousbatteries, and as reinforcing components for polymeric composites. U.S.Pat. Nos. 5,149,584 and 5,618,875 to Baker et al. teach carbonnanofibers as reinforcing components in polymer reinforced composites.The carbon nanofibers can either be used as is, or as part of astructure comprised of carbon fibers having carbon nanofibers growntherefrom.

[0005] U.S. Pat. No. 5,413,866 to Baker et al. teaches carbonnanostructures characterized as having: (i) a surface area from about 50m²/g to 800 m²/g; (ii) an electrical resistivity from about 0.3 μohm·mto 0.8 μohm·m; (iii) a crystallinity from about 5% to about 100%; (iv) alength from about 1 μm to about 100 μm; and (v) a shape that is selectedfrom the group consisting of branched, spiral, and helical. These carbonnanostructures are taught as being prepared by depositing a catalystcontaining at least one Group IB metal and at least one other metal on asuitable refractory support, then subjecting the catalyst-treatedsupport to a carbon-containing gas at a temperature from thedecomposition temperature of the carbon-containing gas to thedeactivation temperature of the catalyst.

[0006] U.S. Pat. No. 5,458,784 also to Baker et al. teaches the use ofthe carbon nanostructures of U.S. Pat. No. 5,413,866 for removingcontaminants from aqueous and gaseous steams; and U.S. Pat. Nos.5,653,951 and 6,159,538 to Rodriguez et al. disclose and claim methodsof incorporating hydrogen into layered nanostructure materialscharacterized as having: (i) crystalline regions; (ii) intersticeswithin the crystalline regions which interstices are from about 0.335 nmto 0.67 nm, and (iii) surfaces of said nanostructure which define theinterstices, which surfaces have chemisorption properties with respectto hydrogen. All of the above referenced U.S. patents are incorporatedherein by reference.

[0007] Carbon nanostructures, particularly carbon nanofibers, aretypically produced by growing them from suitable supported orunsupported powdered metal catalysts at elevated temperatures, in thepresence of hydrogen and an effective decomposing carbon-containingcompound. Typically, the carbon-containing compound is selected from CO,methane, ethane, ethylene, acetylene, propane, propylene, butane,butene, butadiene, pentane, etc. While such a method is currently usedto produce carbon nanostructures in substantial yields, the width of thenanostructures is difficult to control. Narrow width nanostructures aredesirable. For example, the average width of a carbon nanostructure isdependent on the average size of the metal catalytic particle from whichit was grown. This size typically ranges from about 25 to 450 nm.

[0008] One attempt to overcome this shortcoming of controlling carbonnanostructure width was to disperse catalytic metal particles over asuitable substrate, such as an amorphous carbon film, in order toproduce carbon nanostructures having a more uniform narrower width. Thiswas achieved to some degree since a more uniform catalyst particle sizedispersion was achieved. Although the resulting carbon nanostructuresresulting from such a method were found to have an average width abouthalf that of those produced by more conventional techniques at thattime, the yield of nanostructures was vastly reduced and unacceptable.In addition, the support material becomes an added impurity that shouldbe avoided when such a method is used.

[0009] Thus, there is a need in the art for methods for producing highyields of carbon nanostructures, especially carbon nanofibers, having asubstantially uniform narrow width.

SUMMARY OF THE INVENTION

[0010] In accordance with the present invention there is provided amethod for producing powdered metal catalysts for use in the productionof graphitic carbon nanostructures, which method comprises:

[0011] mixing: a) one or more metal compounds selected from the groupconsisting of metal carbonates, metal nitrates, and metal hydroxides,wherein at least one of the metals is a Groups VIII metal, with b) oneor more dispersing agents characterized as: i) being substantially inertwith respect to reaction with the carbon-containing gas at temperaturesup to at least about 750° C.; (ii) being substantially inert withrespect to chemical interaction with the catalytic metals attemperatures up to at least about 750° C.; (iii) not having adeleterious effect on the catalytic activity of the catalytic metals;and (iv) maintaining their physical integrity at temperatures up to atleast about 750° C.;

[0012] calcning the resulting mixture at a temperature from about 200°C. to about 400° C. for an effective amount of time to convert the oneor more metal components to their respective oxide;

[0013] milling the calcined mixture for an effective amount of time todecrease the particles comprising the mixture to a predetermined size;

[0014] chemically reducing the milled, calcined mixture of particleswith hydrogen for an effective amount of time and temperature to reduceat least a portion of the catalytic metal oxides to the metallic state.

[0015] In a preferred embodiment the dispersing agent is selected fromthe group consisting of alkaline halides, alkaline-earth halides, andmetal oxides.

[0016] In another preferred embodiment of the present invention thedispersing agent is removed from the product carbon nanostructure by useof a dilute acid.

[0017] In still another preferred embodiment the dispersing agent isadded in the form of an alkaline halide to the calcined metal oxidemixture.

[0018] In yet another preferred embodiment of the present inventionsubstantially all of the dispersing agent is removed from the productcarbon nanostructures.

[0019] Also in accordance with the present invention there is provided amethod for producing powdered metal catalysts for use in the productionof graphitic carbon nanostructures, which method comprises:

[0020] calcining one or more metal compounds selected from the groupconsisting of metal carbonates, metal nitrates, and metal hydroxides,wherein at least one of the metals is a Groups VIII metal, at atemperature from about 200° C. to about 400° C. for an effective amountof time to convert the one or more metal components to their respectiveoxide;

[0021] adding an effective amount of dispersing agent to the calcinedmetal compound, which dispersing agent is characterized as: i) beingsubstantially inert with respect to reaction with the carbon-containinggas at temperatures up to at least about 750° C.; (ii) beingsubstantially inert with respect to chemical interaction with thecatalytic metals at temperatures up to at least about 750° C.; (iii) nothaving a deleterious effect on the catalytic activity of the catalyticmetals; and (iv) maintaining their physical integrity at temperatures upto at least about 750° C.;

[0022] milling the calcined metal compound and dispersing agent for aneffective amount of time to decrease the particles comprising themixture to a predetermined size;

[0023] chemically reducing the milled mixture of particles with hydrogenfor an effective amount of time and temperature to reduce at least aportion of the catalytic metal oxides to the metallic state.

[0024] Also in accordance with the present invention a suitablecarbon-containing compound having up to about 8 carbon atoms isdecomposed in the presence of at least a portion of the reduced milled,calcined mixture of catalytic metal particles and dispersing agent at atemperature from about 450° C. to about 800° C.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The propensity for carbon nanostructures to be formed during theinteraction of carbon-containing compounds, such as hydrocarbons andcarbon monoxide with hot metal surfaces is known. It has been recognizedin recent years that a unique set of chemical and physical propertiescan be achieved if one controls the growth and structuralcharacteristics of carbon nanostructures by the use of selectedcatalysts. The unusual properties exhibited by carbon nanostructuredmaterials, coupled with the possibility of tailoring these properties aswell as their dimension, have an impact on research activitiesassociated with such carbon nanostructures. Of particular importance arecarbon nanostructures having a relatively high-graphite content andnarrow widths, since such nanostructures have a variety of potentialcommercial applications. Unfortunately, conventional methods forproducing carbon nanostructures are not suitable for producing highyields of carbon nanostructures having relatively uniform narrow widths.The width of carbon nanostructures is typically dictated by the size ofthe catalytic metal particles from which they are grown, which istypically range from about 25 to 450 nm. Better control of, and narrowerwidth nanostructures are highly desirable.

[0026] Non-limiting examples of preferred carbon nanostructures aremulti-walled structures selected from carbon nanotubes, carbonnanoshells, carbon fibrils, and carbon nanofibers. Typically, it isdesirable that the carbon nanostructure be graphitic, and in the case ofcarbon nanofibers, the most preferred carbon nanostructure, theinterstices, or the distance between graphitic platelets, will be about0.335 nm. It is to be understood that the terms “carbon filaments”,“carbon whiskers”, “carbon nanofibers”, and “carbon fibrils”, aresometimes used interchangeably by those having ordinary skill in theart. For purposes of the present invention, carbon fibrils, whichthemselves are sometimes referred to as carbon nanotubes, are of thetype described in U.S. Pat. Nos. 4,663,230 and 5,165,909, both of whichare incorporated herein by reference. Carbon fibrils are essentiallycylindrical discrete structures characterized by a substantiallyconstant diameter between about 3.5 nm and 70 nm, a length greater thanabout 10² times the diameter, an outer region of multiple essentiallycontinuous layers of ordered carbon atoms, and a distinct inner coreregion. Each of the layers and core are reported in the above patents tobe disposed substantially concentrically about the cylindrical axis ofthe fibril. Carbon nanotubes, other than those that are sometimes alsoreferred to as carbon fibrils, will typically be of the fullerene type.Such structures are described in an article by M. S. Dresselhaus et. al.entitled Fullerenes, on pages 2087-2092; Journal of Materials Research,Vol 8, No.8, August 1993 and is incorporated herein by reference.

[0027] Carbon nanoshells, also sometimes referred to as carbonnanoparticles, are typically polyhedral-layered structures comprised ofmultiple layers of carbon, forming substantially closed shells aroundvoids or metal particles of various shapes and sizes. For purposes ofthe present invention, a metal that is capable of dissociativelyabsorbing hydrogen, such as lanthanum and magnesium, is incorporatedinto the void, or hollow inner core of the carbon nanoshell.

[0028] The most preferred carbon nanostructure for purposes of thepresent invention are graphitic nanofibers. These carbon nanofibers arenovel materials having a unique set of properties that include: (i) asurface area from about 20 to 3,000 m²/g, preferably from about 50 to800 m²/g, more preferably from about 100 to 700 m²/g, and mostpreferably from about 250 to 350 m²/g, which surface area is determinedby N₂ adsorption at—196° C.; (ii) a crystallinity from about 5% to about100%, preferably from about 50% to 100%, more preferably from about 75%to 100%, most preferably from about 90% to 100%, and ideallysubstantially 100%; and (iii) interstices of about 0.335 nm to about0.40 nm, preferably about 0.335 nm. The interstices are the distancebetween the graphite platelets. The shape of the nanofibers can be anysuitable shape. Non-limiting examples of preferred shapes includestraight, branched, twisted, spiral, helical, and coiled. The graphiticplatelets can be oriented from substantially perpendicular tosubstantially parallel to the longitudinal, or growth, axis of thenanofiber. In the case where the graphitic platelets are orientedsubstantially perpendicular to the growth axis, the carbon nanofibersare sometimes referred to as “platelet”. In the case where the graphiticplatelets are oriented substantially parallel to the growth axis, theresulting nanofibers can be either “ribbon-like” or “multifacetedtubular”. The ribbon-like structures are composed of discontinuous, ornon-linked platelets and can be thought of as a series of sheets alignedsubstantially parallel to each other. The multifaceted tubularnanostructures have parallel graphite platelets linked at an angledifferent than 180°, preferably about 60° so that they form anon-cylindrical multifaceted tubular structure. Carbon nanoribbons, aswell as other preferred carbon nanostructures of the present inventionare those wherein at least about 5%, preferably at least about 50%, morepreferably at least about 80%, and most preferably at least about 95% ofthe edge sites are exposed. Preferred carbon nanoribbon type materialsare those wherein the platelets are continuous to form anon-cylindrical, but multi-faceted tubular structure, somewhat like thestructure of a multi-faced pencil. The graphitic platelets can also beoriented at an angle to the growth axis and thus are sometime referredto as “herringbone”. Further, the surface area of the carbon nanofiberscan be dramatically increased by careful activation with a suitableetching agent, such as carbon dioxide, steam, or the use of selectedcatalyst, such as an alkali or alkaline-earth metal.

[0029] The carbon nanostructures of the present invention arecatalytically grown from unsupported metal powders. In this case, acarbon-containing compound is decomposed in the presence of the metalcatalyst at temperatures from about 450° C. to about 800° C., morepreferably from about 550° C. to about 700° C. It is also preferred thathydrogen be present during the decomposition of the carbon-containingcompound.

[0030] Catalysts suitable for growing the carbon nanostructures of thepresent invention include both single metals, as well as alloys andmulti-metallics. If the catalyst is a single metal then it will be aGroup VIII metal selected from Fe, Ni, and Co. If the catalyst is analloy or multimetallic material, then it is comprised of a first metalcomponent that will be one or more Group VIII metals and a second metalthat is preferably one or more Group IB metals, such as Cu, Ag, and Au.Preferred are Cu and Ag with Cu being the most preferred. If thecatalyst is an alloy or multimetallic it is preferred that the catalystbe comprised of two Group VIII metals or one Group VIII metal and oneGroup IB metal. It will be understood that Zn can be used in place ofone or more of the Group VIII metals. The Group IB metals is present inan amount ranging from about 0.5 to 99 at. % (atomic %). For example thecatalyst can contain up to about 99 at. %, even up to about 70 at. %, oreven up to about 50 at. %, preferably up to about 30 at. %, morepreferably up to about 10 at. %, and most preferably up to about 5 wt. %copper, of Group IB metal with the remainder being a Group VIII metal,preferably nickel or iron, more preferably iron. Catalysts having a highcopper content (70 at. % to 99 at. %) will typically generate nanofiberswhich are predominantly helical or coiled, and which have a relativelylow crystallinity (from about 5 to 25%). Lower concentrations of copper,e.g., 0.5 to 30 at. % have a tendency to produce spiral and branchednanofibers, whereas a catalyst with about 30 to 70 at. %, preferably 30to 50 at. % copper will produce predominantly branched nanofibers. Athird metal can also be present. Although there is no limitation withrespect to what the particular third metal can be, it is preferred thatit be selected from the group consisting of Ti, W, Sn and Ta. When athird metal is present, it is substituted for up to about 20 at. %,preferably up to about 10 at. %, and more preferably up to about 5 at.%, of the second metal. It is preferred that the catalyst be comprisedof Cu in combination with Fe, Ni, or Co. More preferred is Cu incombination with Fe and/or Ni from an economic point of view. A catalystof which Fe is used in place of some of the Ni would be less expensivethan a catalyst comprised of Cu in combination with only Ni.

[0031] Any suitable method can be used to produce the powdered metalcatalyst. As previously mentioned, it is most preferred in the practiceof the present invention that the carbon nanostructures be grown fromunsupported metallic powders. A preferred method for preparing suitableunsupported metal catalytic powders is the use of colloidal techniquesfor precipitating them as metal oxides, hydroxides, carbonates,carboxylates, nitrates, etc. Such a process typically involvesdissolving salts of each metal of the catalyst in an appropriatesolvent, preferably water. A suitable precipitating agent, such as anammonium carbonate, ammonium bicarbonate or ammonium hydroxide is addedto the solution, thereby causing the metal to precipitate out as thecorresponding metal carbonate or hydroxide. The precipitate is thendried at a temperature greater than about 100° C., preferably from about105° C. to about 120° C., and more preferably at about 110° C. Afterdrying, the precipitate is mixed with a suitable dispersing agent andcalcined at a temperature from about 200° to 400° C., preferably fromabout 200° to about 300° C., thereby converting the individual metals totheir respective oxide form. Alternatively, the dispersing agent can beincorporated after calcination. The mixed oxide forms, together with thedispersing agent, are then milled, preferably ball milled, undersuitable conditions, to produce a dispersed metal powder catalyst ofdesired size for carbon nanostructure growth. The milled metal powdermixture is then heated, in a hydrogen-containing atmosphere, at atemperature from about 400° to about 600° C., preferably from about 450°to 550° C., for an effective amount of time, to produce the catalyst inits metallic state. The dispersing agent may also be reduced to itsmetallic state, depending on the dispersing agent. For example, if thedispersing agent is a metal oxide it can be reduced to its metallicstate, whereas if the dispersing agent is a salt, such as sodiumchloride, it will remain as is during treatment with a hydrogenatmosphere. By effective amount of time, we mean that amount of timeneeded to reduce substantially all of the metal oxides to the respectivemetal or alloy having a suitable particle size. A typical amount of timewill generally be from about 15 to 25 hours. Suitable particle sizes arefrom about 2.5 nm to about 150 nm, preferably from about 2.5 nm to about100 nm, and more preferably from about 2.5 nm to about 20 nm. Followingthis treatment the chemically reduced catalyst is cooled to about roomtemperature in a helium environment before being passivated in a 2%oxygen/helium mixture for 1 hour at about room temperature (24° C.).

[0032] Salts of the catalytic metal suitable for use in the presentinvention are salts that are soluble in both water, organic solvents,and diluted mineral acids. Non-lmiting examples of water-soluble saltssuitable for use herein include nitrates, sulfates and chlorides.Non-limiting examples of preferred salts soluble in organic solvents,which are suitable for use herein, include formates, acetates, andoxalates. Non-lmiting examples of organic solvents that are suitable foruse herein include alcohols, such as methanol, ethanol, propanol, andbutanol; ketones, such as acetone; acetates and esters; and aromatics,such as benzene and toluene.

[0033] Dispersing agents suitable for use in the present invention arethose that: (a) are substantially inert with respect to reaction withthe carbon-containing gas at temperatures up to at least about 750° C.;(b) are substantially inert with respect to chemical interaction withthe catalytic metals at temperatures up to at least about 750° C.; (c)do not have a deleterious effect on the catalytic activity of thecatalytic metals; and (d) maintain their physical integrity attemperatures up to at least about 750° C. Dispersing agents are to bedistinguished from supports. In a typical supported catalyst system, theamount of support is far greater than the amount of the metal component.That is, the weight of support versus catalytic metal on support isgenerally greater than about 50 wt. %, typically much greater than about85 wt. %, based on the total weight of support plus catalytic metal. Incontrast, the amount of dispersing agent used in the practice of thepresent invention will be substantially lower than the amount of metal.The amount of dispersing agent used in the practice of the presentinvention will be from about 1 wt. % to about 50 wt. %, preferably fromabout 5 wt. % to about 25 wt. %, and more preferably from about 5 wt. %to about 10 wt. %, based on the total weight of catalytic metal plusdispersing agent. It is preferred that only an effective amount ofdispersing agent be used. That is, only that amount needed to preventthe particles of the powdered metal catalyst from sintering oragglomerating.

[0034] Preferred classes of compounds that can be used as dispersingagents in the practice of the present invention include alkaline andalkaline earth halides and metal oxides. Non-limiting examples ofalkaline halides that can be used in the practice of the presentinvention include: sodium fluoride, sodium chloride, sodium bromide,potassium fluoride, potassium chloride, potassium bromide, lithiumfluoride, and rubidium fluoride; preferred are sodium chloride andpotassium chloride, and more preferred is sodium chloride. Non-limitingexamples of alkaline earth halides that can be used in the practice ofthe present invention include calcium fluoride, calcium chloride,calcium bromide, magnesium fluoride, magnesium chloride, magnesiumbromide, barium fluoride, barium chloride, barium bromide, strontiumfluoride, strontium chloride and strontium bromide; preferred arecalcium chloride and magnesium chloride, and more preferred is calciumchloride. Non-limiting examples of preferred metal oxides includemagnesia, silica, alumina, titania, tungsten oxide, tantalum oxide,molybdenum oxide, lanthanum oxide, tellurium oxide, chromium oxide,niobium oxide and zirconium oxide. Preferred are magnesia, silica, andtitania; and more preferred is silica.

[0035] As previously mentioned, one or more dispersing agents is mixedwith one or more catalytic metal precursor compounds prior to, orfollowing calcining. The catalytic metal precursor compound will mosttypically be a carbonate, nitrate, or hydroxide of the desired catalyticmetal. The mixture is then calcined as previous mentioned at atemperature from about 200° C. to about 400° C. to produce therespective oxide forms of the catalytic metal. The calcining or coursewill be preformed in an oxidizing atmosphere, preferably in air. Thismixture of oxides, catalytic metal oxides plus dispersant oxide, arethen subjected to any suitable technique that will reduce their particlesize. A preferred size reduction technique is milling, preferably ballmilling, to the size desired for catalytic growth of carbonnanostructures. It is preferred that the milled particle size be fromabout 2.5 nm to about 150 nm, preferably from about 2.5 nm to about 100nm, and more preferably from about 2.5 nm to about 20 nm. Typicalmilling times will be from about 24 hrs to about 72 hrs. The presence ofthe dispersing agent during milling keeps the metal powder particlesfrom sintering, or agglomerating, during the milling process. Thedispersing agent can be irreversibly oxidized, or present as a stablealkaline halide, and remains in that state within the metal granulesfollowing subsequent reduction, thus minimizing sintering of themetallic compounds of the catalyst. This enables the production ofpowder catalyst particles substantially smaller than those obtained byconventional techniques that do not employ the use of a dispersingagent.

[0036] In a more preferred embodiment, the dispersing agent is a solidthat remains substantially stable in its initial chemical state whentreated in the presence of either oxygen or hydrogen at temperatures upto at least about 750° C. In the most preferred embodiment, thedispersing agent is removed from the system after the catalyst has beenused to grow carbon nanofibers. This step can be accomplished bytreatment with water, dilute mineral acid or dilute alkali solution. Inthis way the dispersing agent is not present in the final solid carbonnanostructure product.

[0037] Carbon-containing compounds suitable for use in the practice ofthe present invention are compounds composed mainly of carbon atoms andhydrogen atoms, although carbon monoxide can also be used. Thecarbon-containing compound, which is typically introduced into theheating zone in gaseous form, will generally have no more than 8 carbonatoms, preferably no more than 6 carbon atoms, more preferably no morethan 4 carbon atoms, and most preferably no more than 2 carbon atoms.Non-limiting examples of such compounds include CO, methane, ethane,ethylene, acetylene, propane, propylene, butane, butene, butadiene,pentane, pentene, cyclopentadiene, hexane, cyclohexane, benzene,toluene. Combinations of gases are preferred, particularly carbonmonoxide and ethylene.

[0038] It may be desirable to have an effective amount of hydrogenpresent in the heating, or growth, zone during nanostructure growth.Hydrogen serves two complementary functions. For example, on the onehand it acts as a cleaning agent for the catalyst, and on the other handit hydrogasifies, or causes carbon burn-off, of the carbon structure. Byan effective amount, we mean that minimum amount of hydrogen that willmaintain a clean catalyst surface (free of carbon residue), but not somuch that will cause excessive hydrogasification, or burn-off, of carbonfrom the nanostructures and/or substrate structure, if present.Generally, the amount of hydrogen present will range from about 5 to 40vol. %, preferably from about 10 to 30 vol. %, and more preferably fromabout 15 to 25 vol. %. For some catalyst systems, such as Cu:Fe, thehydrogasification reaction is relatively slow, thus, an effective amountof hydrogen is needed to clean the catalyst in order to keep it clean ofcarbon residue and maintain its activity. For other catalyst systems,such as Cu:Ni, where the activity is so high that excessivehydrogasification occurs, even at relatively low levels of hydrogen,little, if any, hydrogen is needed in the heating zone. A Cu:Ni catalystis so active that it utilizes essentially all of the carbon depositedthereon to grow nanofibers, and thus, there is generally no carbonresidue to clean off.

[0039] After the carbon nanostructures, preferably nanofibers, aregrown, it may be desirable to treat the final structure with an aqueoussolution of an inorganic acid, such as a mineral acid, to remove anyexcess catalyst particles. Non-limiting examples of mineral acids, whichcan be used, include sulfuric acid, nitric acid, and hydrochloric acid.Preferred is hydrochloric acid.

[0040] The edges of the graphite platelets may be etched with anappropriate etching agent, preferably carbon dioxide, steam, or asuitable catalyst such as an alkali or alkaline-earth metal.

[0041] The present invention will be illustrated in more detail withreference to the following examples, which should not be construed to belimiting in scope of the present invention.

EXAMPLE 1

[0042] In the first series of experiments various amounts of the siliconcompound, tetraethyl-orthosilicate, were added to a mixture of iron andnickel carbonates and subsequently calcined at 400° C. for 4 hours in100 ml/min air. The resulting mixed oxide powder was removed from thereactor, ball-milled for 60 hours, then reduced in a 10% H₂/Heatmosphere for 20 hours at 500° C. Upon cooling to room temperature, thecatalyst was passivated in a 2% air/He flow for one hour. Under theseconditions, the iron and nickel components were reduced to the metallicstate, but the silicon remained in the fully oxidized state.

[0043] These catalyst systems were used to synthesize graphitenanofibers from the interaction of a CO/H₂ (1:4) mixture at 670° C. Thewidth distributions of the graphite nanofiber obtained from theseexperiments was obtained from measurements performed in a transmissionelectron microscope. The data shown in Table 1 below show the variationin average widths of graphite nanofibers grown from Fe—Ni (6:4)-CO/H₂(1:4) at 670° C. for 3.0 hours as a function of the concentration ofSiO₂ additive. Inspection of this data evidences that the narrowestgraphite nanofibers are obtained with a catalyst containing 5 wt. % ofSiO₂ additive. TABLE 1 % SiO₂ Additive in Fe- Average GNF Ni (6:4)catalyst Width (nm) 0 34.7 1 32.7 5 23.0 15 31.2

EXAMPLE 2

[0044] The procedure of Example 1 was followed except various amounts ofsodium chloride were added to a Fe—Ni (6:4) catalyst and the resultingcatalyst system used to grow graphite nanofibers from the interactionwith a CO—H₂ (1:4) mixture at 670° C. The data presented in Table 2below show the variation in average widths of graphite nanofibers grownfrom Fe—Ni (6:4)-CO/H₂ (1:4) at 670° C. for 4.0 hours as a function ofthe concentration of the NaCl additive. Inspection of this data showsthat the narrowest graphite nanofibers are obtained from this systemwith a catalyst containing 5 to 10 wt. % of NaCl additive. TABLE 2 %NaCl Additive in Fe- Average GNF Ni (6:4) Catalyst Width (nm) 0 27.7 2.520.4 5.0 11.6 10.0 11.7 15.0 28.0

EXAMPLE 3

[0045] The procedure of Example 1 was followed except 5 wt. % sodiumchloride was added to a Fe—Ni (6:4) catalyst and the resulting catalystsystem was used to grow graphite nanofibers from the interaction with aCO—H₂ (1:4) mixture at various temperatures. The data given in Table 3below shows the variation in average widths of graphite nonofibers grownfrom 5 wt. % NaCl Fe—Ni (6:4)-CO/H₂ (1:4) at temperatures ranging from650 to 675° C. for 3.0 hours. It is evident that for this catalystcomposition the narrowest graphite nanofibers are obtained attemperatures between 665 and 675° C. TABLE 3 Average GNF WidthTemperature (° C.) (nm) 650 53.9 660 28.7 665 18.0 670 16.5 675 17.3

EXAMPLE 4

[0046] The procedure of Example 1 was followed except 5 wt. % sodiumchloride was added to a Fe—Ni 6:4) catalyst and the resulting catalystsystem was used to grow graphite nonofibers from the interaction with aCO—H₂ (1:4) mixture at 670° C. for various periods of time. The datapresented in Table 4 below shows the variation in average width and theamounts of nanofibers formed as a function of reaction time. TABLE 4Average GNF Width Grams GNF/Grams Time (hours) (nm) Metal 3 16.5 11.32 411.6 14.02 5 11.6 20.89 12 16.2 28.79

EXAMPLE 5

[0047] During the interaction of Fe—Ni (6:4) with CO/H₂ (1:4) at 660 to675° C. in the addition to the growth of graphite nanofibers there isalso a significant fraction of an undesirable “shell” type materialformed. The procedure of Example 1 was followed except catalyst powderscontaining 2.5 to 15 wt. % NaCl were used and the effect of certainpost-reaction treatments on the formation of the “shell” type depositsresulting from the Fe—Ni (6:4) catalytic decomposition of CO/H₂ (1:4) at670° C. for 5 hours was examined. From the data presented in Table 5below it is evident that one can reduce the fraction of this undesirablecomponent in all catalysts containing NaCl by continuing to heat thesample in a H₂/He (4:1) mixture at 670° C. for 1.0 hours after the COhas been switched off and then cooling in He to room temperature. TABLE5 % “shells” Additive Post-treatment (>78 nm) None None 16.83 2.5% NaCl H₂/He (4:1) at 670° C. for 1.0 hr 4.30  5% NaCl H₂/He (4:1) at 670° C.for 1.0 hr 3.96 10% NaCl H₂/He (4:1) at 670° C. for 1.0 hr 2.44 10% NaClH₂/He (4:1) at 670° C. for 1.0 hr 0.02 15% NaCl H₂/He (4:1) at 670° C.for 1.0 hr 3.33

EXAMPLE 6.

[0048] The procedure of Example 1 was followed except catalyst powderscontaining 2.5 to 15 wt. % NaCl were used and the effect of certainpost-reaction treatments on the growth characteristics of graphitenanofibers produced from the Fe—Ni (6:4) catalytic decomposition ofCO/H₂ (1:4) at 670° C. for 5 hours was examined. The data presented inTable 6 below shows the variation in average width and the amounts ofnanofibers formed as a function of the amount of added NaCl in thecatalyst preparation. Inspection of these results indicates that onceagain the narrowest width is obtained from catalyst formulationscontaining 5 to 10 wt. % NaCl. TABLE 6 Average GNF AdditivePost-treatment Width (nm) 2.5% NaCl  H₂/He (4:1) at 670° C. for 1.0 hr14.0  5% NaCl H₂/He (4:1) at 670° C. for 1.0 hr 11.6 10% NaCl H₂/He(4:1) at 670° C. for 1.0 hr 9.4 10% NaCl H₂/He (4:1) at 670° C. for 1.0hr 9.8 15% NaCl H₂/He (4:1) at 670° C. for 1.0 hr 16.0

1. A method for producing powdered metal catalyst for use in theproduction of carbon nanostructures, which method comprises: mixing a)one or more metal compounds selected from the group consisting of metalcarbonates, metal nitrates, and metal hydroxides, wherein at least oneof the metals is a Groups VIII metal, with b) one or more dispersingagents characterized as: i) being substantially inert with respect toreaction with the carbon-containing gas at temperatures up to at leastabout 750° C.; (ii) being substantially inert with respect to chemicalinteraction with the catalytic metals at temperatures up to at leastabout 750° C.; (iii) not having a deleterious effect on the catalyticactivity of the catalytic metals; and (iv) capable of maintaining theirphysical integrity at temperatures up to at least about 750° C.;calcining the resulting mixture at a temperature from about 200° C. toabout 400° C. for an effective amount of time to convert at least themetal compound to its respective oxide; milling the calcined mixture foran effective amount of time to decrease the particles comprising themixture to a predetermined size; treating the milled, calcined mixtureof particles with hydrogen for an effective amount of time andtemperature to chemically reduce at least a portion of the catalyticmetal oxides to the metallic state.
 2. The method of claim 1 wherein theamount of dispersing agent used is from about 1 to 50 wt. %, based onthe total amount of catalytic metal compound and dispersing agent. 3.The method of claim 2 wherein the amount of dispersing agent used isfrom about 5 to 25 wt. %
 4. The method of claim 3 wherein the amount ofdispersing agent used is from about 5 to about 10 wt. %. 5 The method ofclaim 1 wherein the dispersing agent is selected from the groupconsisting of alkaline halides, alkaline-earth halides, and metaloxides.
 6. The method of claim 5 wherein the dispersing agent is analkaline halide selected from the group consisting of sodium fluoride,sodium chloride, sodium bromide, potassium fluoride, potassium chloride,potassium bromide, lithium fluoride, and rubidium fluoride.
 7. Themethod of claim 6 wherein the dispersing agent is sodium chloride. 8.The method of claim 5 wherein the dispersing agent is an alkaline-earthhalide selected from the group consisting of calcium fluoride, calciumchloride, calcium bromide, magnesium fluoride, magnesium chloride,magnesium bromide, barium fluoride, barium chloride, barium bromide,strontium fluoride, strontium chloride and strontium bromide; preferredare calcium chloride and magnesium chloride, and more preferred iscalcium chloride.
 9. The method of claim 5 wherein the dispersing agentis a metal oxide selected from the group consisting of magnesia, silica,alumina, titania, tungsten oxide, tantalum oxide, molybdenum oxide,lanthanum oxide, tellurium oxide, chromium oxide, niobium oxide andzirconium oxide.
 10. The method of claim 1 wherein the predeterminedsize is about 2.5 nm to about 100 nm.
 11. The method of claim 1 whereinthe catalytic metal is a bimetallic comprised of iron and nickel. 12.The method of claim 1 wherein the catalytic metal is a multimetalliccomprised of at least one Group VIII metal and at least one Group IBmetal.
 13. The method of claim 12 wherein the catalytic metal is abimetallic comprised of iron and copper.
 14. A method for producingpowdered metal catalyst for use in the production of carbonnanostructures, which method comprises: calcining one or more metalcompounds selected from the group consisting of metal carbonates, metalnitrates, and metal hydroxides, wherein at least one of the metals is aGroups VIII metal at a temperature from about 200° C. to about 400° C.for an effective amount of time to convert at least the metal compoundto its respective oxide; adding an effective amount of dispersing agentto the calcined metal compound, which dispersing agent is characterizedas: i) being substantially inert with respect to reaction with thecarbon-containing gas at temperatures up to at least about 750° C.; (ii)being substantially inert with respect to chemical interaction with thecatalytic metals at temperatures up to at least about 750° C.; (iii) nothaving a deleterious effect on the catalytic activity of the catalyticmetals; and (iv) maintaining their physical integrity at temperatures upto at least about 750° C.; milling the calcined metal compound anddispersing agent for an effective amount of time to decrease theparticles comprising the mixture to a predetermined size; chemicallyreducing the milled mixture of particles with hydrogen for an effectiveamount of time and temperature to reduce at least a portion of thecatalytic metal oxides to the metallic state.
 15. The method of claim 14wherein the amount of dispersing agent used is from about 1 to 50 wt. %,based on the total amount of catalytic metal precursor and dispersingagent.
 16. The method of claim 15 wherein the amount of dispersing agentused is from about 5 to 25 wt. %
 17. The method of claim 16 wherein theamount of dispersing agent used is from about 5 to about 10 wt. %. 18.The method of claim 14 wherein the dispersing agent is selected from thegroup consisting of alkaline halides, alkaline-earth halides, and metaloxides.
 19. The method of claim 18 wherein the dispersing agent is analkaline halide selected from the group consisting of sodium fluoride,sodium chloride, sodium bromide, potassium fluoride, potassium chloride,potassium bromide, lithium fluoride, and rubidium fluoride.
 20. Themethod of claim 19 wherein the dispersing agent is sodium chloride. 21.The method of claim 18 wherein the dispersing agent is an alkaline-earthhalide selected from the group consisting of calcium fluoride, calciumchloride, calcium bromide, magnesium fluoride, magnesium chloride,magnesium bromide, barium fluoride, barium chloride, barium bromide,strontium fluoride, strontium chloride and strontium bromide; preferredare calcium chloride and magnesium chloride, and more preferred iscalcium chloride.
 22. The method of claim 19 wherein the dispersingagent is a metal oxide selected from the group consisting of magnesia,silica, alumina, titania, tungsten oxide, tantalum oxide, molybdenumoxide, lanthanum oxide, tellurium oxide, chromium oxide, niobium oxideand zirconium oxide.
 23. The method of claim 14 wherein thepredetermined size is about 2.5 nm to about 100 nm.
 24. The method ofclaim 14 wherein the catalytic metal is a bimetallic comprised of ironand nickel.
 25. The method of claim 14 wherein the catalytic metal is amultimetallic comprised of at least one Group VIII metal and at leastone Group IB metal.
 26. The method of claim 14 wherein the catalyticmetal is a bimetallic comprised of iron and nickel.
 27. A method forproducing a carbon nanostructure from a powdered metal catalyst, whichmethod comprises: mixing a) one or more metal compounds selected fromthe group consisting of metal carbonates, metal nitrates, and metalhydroxides, wherein at least one of the metals is a Groups VIII metal,with b) one or more dispersing agents characterized as: i) beingsubstantially inert with respect to reaction with the carbon-containinggas at temperatures up to at least about 750° C.; (ii) beingsubstantially inert with respect to chemical interaction with thecatalytic metals at temperatures up to at least about 750° C.; (iii) nothaving a deleterious effect on the catalytic activity of the catalyticmetals; and (iv) capable of maintaining their physical integrity attemperatures up to at least about 750° C.; calcining the resultingmixture at a temperature from about 200° C. to about 400° C. for aneffective amount of time to convert at least the metal compound to itsrespective oxide; milling the calcined mixture for an effective amountof time to decrease the particles comprising the mixture to apredetermined size; treating the milled, calcined mixture of particleswith hydrogen for an effective amount of time and temperature tochemically reduce at least a portion of the catalytic metal oxides tothe metallic state; and decomposing a carbon-containing compound havingup to about 8 carbon atoms in the presence of at least a portion of thereduced milled, calcined mixture of catalytic metal particles anddispersing agent at a temperature from about 450° C. to about 800° C.28. The method of claim 27 wherein the amount of dispersing agent usedis from about 1 to 50 wt. %, based on the total amount of catalyticmetal compound and dispersing agent.
 29. The method of claim 28 whereinthe amount of dispersing agent used is from about 5 to 25 wt. %
 30. Themethod of claim 29 wherein the amount of dispersing agent used is fromabout 5 to about 10 wt. %. 31 The method of claim 27 wherein thedispersing agent is selected from the group consisting of alkalinehalides, alkaline-earth halides, and metal oxides.
 32. The method ofclaim 31 wherein the dispersing agent is an alkaline halide selectedfrom the group consisting of sodium fluoride, sodium chloride, sodiumbromide, potassium fluoride, potassium chloride, potassium bromide,lithium fluoride, and rubidium fluoride.
 33. The method of claim 32wherein the dispersing agent is sodium chloride.
 34. The method of claim31 wherein the dispersing agent is an alkaline-earth halide selectedfrom the group consisting of calcium fluoride, calcium chloride, calciumbromide, magnesium fluoride, magnesium chloride, magnesium bromide,barium fluoride, barium chloride, barium bromide, strontium fluoride,strontium chloride and strontium bromide; preferred are calcium chlorideand magnesium chloride, and more preferred is calcium chloride.
 35. Themethod of claim 31 wherein the dispersing agent is a metal oxideselected from the group consisting of magnesia, silica, alumina,titania, tungsten oxide, tantalum oxide, molybdenum oxide, lanthanumoxide, tellurium oxide, chromium oxide, niobium oxide and zirconiumoxide.
 36. The method of claim 27 wherein the predetermined size isabout 2.5 nm to about 100 nm.
 37. The method of claim 27 wherein thecatalytic metal is a bimetallic comprised of iron and nickel.
 38. Themethod of claim 27 wherein the catalytic metal is a multimetalliccomprised of at least one Group VIII metal and at least one Group IBmetal.
 39. The method of claim 38 wherein the catalytic metal is abimetallic comprised of iron and copper.
 40. The method of claim 37wherein the ratio of iron to nickel is from about 1:9 to about 9:1. 41.The method of claim 40 wherein the ratio of iron to nickel is from about3:7 to about 7:3
 42. The method of claim 40 wherein the ratio of CO toH₂ is from about 95:5 to about 5:95.
 43. The method of claim 42 whereinthe ratio of CO to H₂ is from about 80:20 to about 20:80.
 44. The methodof claim 40 wherein the ratio of CO to H₂ is from about 80:20 to about20:80.
 45. The method of claim 14 wherein the particle size of thecatalytic metal powder is from about 0.5 nanometer to about 5micrometer.
 46. The method of claim 45 wherein the particle size of thebimetallic powder is from about 2.5 nanometer to about 1 micrometer. 47.The method of claim 46 wherein the ratio of hydrogen to helium is about4 to
 1. 48. The method of claim 27 wherein the dispersing agent isremoved from the resulting carbon nanofibers.
 49. The method of claim 48wherein the dispersing agent is removed from the resulting carbonnanofibers by dissolving the dispersing agent in a suitable solventselected from the group consisting of water, dilute acid, or with adilute alkali solution.
 50. The method of claim 14 that includes a stepof decomposing a carbon-containing compound having up to about 8 carbonatoms in the presence of at least a portion of the milled and reducedmixture of catalytic metal particles and dispersing agent at atemperature from about 450° C. to about 800° C.